1. Field of the Invention
The present invention relates to an ink-jet printhead. More particularly, the present invention relates to a thermally driven monolithic ink-jet printhead in which a nozzle plate, including a tapered nozzle, is formed integrally with a substrate and a method for manufacturing the same.
2. Description of the Related Art
In general, ink-jet printheads are devices for printing a predetermined image, color or black, by ejecting a small volume droplet of a printing ink at a desired position on a recording sheet. Ink-jet printheads are largely classified into two types depending on the ink droplet ejection mechanisms: a thermally driven ink-jet printhead, in which a heat source is employed to form and expand a bubble in ink thereby causing an ink droplet to be ejected, and a piezoelectrically driven ink-jet printhead, in which a piezoelectric crystal bends to exert pressure on ink causing an ink droplet to be expelled.
An ink droplet ejection mechanism of a thermally driven ink-jet printhead will now be described in detail. When a pulse current flows through a heater formed of a resistive heating material, heat is generated by the heater. The heat causes ink near the heater to be rapidly heated to approximately 300° C., thereby boiling the ink and generating a bubble in the ink. The formed bubble expands and exerts pressure on ink contained within an ink chamber. This pressure causes a droplet of ink to be ejected through a nozzle from the ink chamber.
A thermally driven ink-jet printhead can be further subdivided into top-shooting, side-shooting, and back-shooting type depending on the direction in which the ink droplet is ejected and the direction in which a bubbles expands. While the top-shooting type refers to a mechanism in which an ink droplet is ejected in a direction the same as a direction in which a bubble expands, the back-shooting type is a mechanism in which an ink droplet is ejected in a direction opposite to a direction in which a bubble expands. In the side-shooting type, the direction of ink droplet ejection is perpendicular to the direction of bubble expansion.
Thermally driven ink-jet printheads need to meet the following conditions. First, a simple manufacturing process, low manufacturing cost, and mass production must be provided. Second, to produce high quality color images, the distance between adjacent nozzles must be as small as possible while still preventing cross-talk between the adjacent nozzles. More specifically, to increase the number of dots per inch (DPI), many nozzles must be arranged within a small area. Third, for high-speed printing, a cycle beginning with ink ejection and ending with ink refill must be as short as possible. That is, the heated ink and heater should cool down quickly to increase an operating frequency.
FIG. 1A illustrates a partial cross-sectional perspective view showing a structure of a conventional thermally driven printhead. FIG. 1B illustrates a cross-sectional view of the printhead of FIG. 1A for explaining a process of ejecting an ink droplet.
Referring to FIGS. 1A and 1B, a conventional thermally driven ink-jet printhead includes a substrate 10, a barrier wall 14 disposed on the substrate 10 for defining an ink chamber 26 filled with ink 29, a heater 12 installed in the ink chamber 26, and a nozzle plate 18 having a tapered nozzle 16 for ejecting an ink droplet 29′. If a pulse current is supplied to the heater 12, the heater 12 generates heat to form a bubble 28 due to the heating of the ink 29 contained within the ink chamber 26. The formed bubble 28 expands to exert pressure on the ink 29 contained within the ink chamber 26, which causes an ink droplet 29′ to be ejected through the tapered nozzle 16. Then, the ink 29 is introduced from a manifold 22 through an ink channel 24 to refill the ink chamber 26.
The process of manufacturing a conventional top-shooting type ink-jet printhead configured as above involves separately manufacturing the nozzle plate 18 equipped with the tapered nozzle 16 and the substrate 10 having the ink chamber 26 and the ink channel 24 formed thereon and bonding them to each other. These required steps complicate the manufacturing process and may cause a misalignment during the bonding of the nozzle plate 18 with the substrate 10.
Recently, in an effort to overcome the above problems of the conventional ink-jet printheads, ink-jet printheads having a variety of structures have been proposed. FIGS. 2A and 2B illustrate a conventional monolithic ink-jet printhead. FIGS. 2A and 2B illustrate a plan view showing an example of a conventional monolithic ink-jet printhead and a vertical cross-sectional view taken along line A-A′ of FIG. 2A, respectively.
Referring to FIGS. 2A and 2B, a hemispherical ink chamber 32 and a manifold 36 are formed on a front surface and a rear surface of a silicon substrate 30, respectively. An ink channel 34 is formed at a bottom of the ink chamber 32 and connects the ink chamber 32 with the manifold 36. A nozzle plate 40, including a plurality of material layers 41, 42, and 43 stacked on the substrate 30, is formed integrally with the substrate 30. The nozzle plate 40 has a nozzle 47 formed at a location corresponding to a central portion of the ink chamber 32. A heater 45 connected to a conductor 46 is disposed around the nozzle 47. A nozzle guide 44 extends along an edge of the nozzle 47 toward a depth direction of the ink chamber 32. Heat generated by the heater 45 is transferred through an insulating layer 41 to ink 48 within the ink chamber 32. The ink 48 then boils to form bubbles 49. The formed bubbles 49 expand to exert pressure on the ink 48 contained within the ink chamber 32, which causes an ink droplet 48′ to be ejected through the nozzle 47. Then, the ink 48 flows through the ink channel 34 from the manifold 36 due to surface tension of the ink 48 contacting the air to refill the ink chamber 32.
A conventional monolithic ink-jet printhead configured as above has an advantage in that the silicon substrate 30 is formed integrally with the nozzle plate 40 thereby simplifying the manufacturing process and eliminating the chance of misalignment.
In the monolithic ink-jet printhead shown in FIGS. 2A and 2B, however, it is difficult to make the material layers 41, 42, and 43 of the nozzle plate 40 thick since they are formed by a chemical vapor deposition (CVD) process. That is, since the nozzle plate 40 has a thickness as small as about 5 μm, it is difficult to provide a sufficient length of the nozzle 47. A small length of the nozzle 47 not only decreases the directionality of the ink droplet 48′ ejected but also prohibits stable high speed printing since a meniscus in the surface of the ink 48, which cannot be formed within the nozzle 47 after ejection of the ink droplet 48′, moves within the ink chamber 32. Further, since the nozzle 47 is formed by etching the material layers 41, 42, and 43, it is difficult to form a nozzle 47 having a tapered shape, i.e., having a shape in which a diameter of the nozzle 47 decreases gradually toward an exit thereof.
In an effort to solve these problems, the conventional ink-jet printhead has the nozzle guide 44 formed along the edge of the nozzle 47. However, if the nozzle guide 44 is too long, this not only makes it difficult to form the ink chamber 32 by etching the substrate 30 but also restricts expansion of the bubbles 49. Thus, use of the nozzle guide 44 causes a restriction on sufficiently providing the length of the nozzle 47.
In addition, in the conventional ink-jet printhead, the material layers 41, 42, and 43 disposed around the heater 45 are made from low heat conductive insulating materials, such as an oxide or a nitride, to provide electrical insulation. Thus, a significant time must elapse for the heater 45, the ink 48 within the ink chamber 32, and the nozzle guide 44, all of which are heated for ejection of the ink 48, to sufficiently cool down and return to an initial state, thereby making it difficult to increase an operating frequency of the printhead to a sufficient level.